Barium copper sulfur fluoride transparent conductive thin films and bulk material

ABSTRACT

A bulk barium copper sulfur fluoride (BCSF) material can be made by combining Cu 2 S, BaS and BaF 2 , heating the ampoule between 400 and 550° C. for at least two hours, and then heating the ampoule at a temperature between 550 and 950° C. for at least two hours. The BCSF material may be doped with potassium, rubidium, or sodium. Additionally, a p-type transparent conductive material can comprise a thin film of BCSF on a substrate where the film has a conductivity of at least 1 S/cm. The substrate may be a plastic substrate, such as a polyethersulfone, polyethylene terephthalate, polyimide, or some other suitable plastic or polymeric substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 12/250,808 filed on Oct. 14, 2008 by Jesse A. Frantz et al.,entitled “BARIUM COPPER SULFUR FLUORIDE TRANSPARENT CONDUCTIVE THINFILMS AND BULK MATERIAL,” which claimed priority from U.S. ProvisionalApplication No. 61/098,390 filed on Sep. 19, 2008 by Jesse A. Frantz etal., entitled “BARIUM COPPER SULFUR FLUORIDE TRANSPARENT CONDUCTIVE THINFILMS AND BULK MATERIAL,” the entire contents of each are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to transparent conductivematerials and more particularly to barium copper sulfur fluoridetransparent conductive thin films and bulk materials.

2. Description of the Prior Art

Films consisting of transparent conductors (TC's) with p-type chargecarriers (p-TC's) are desirable for several applications includingmultijunction solar cells, transparent electronics, low operatingvoltage light-emitting diodes, high-efficiency solar cells, andelectro-magnetic interference (EMI) shielding. Moreover, transparentconductive coatings that can be deposited on plastic substrates aredesirable for many applications, for example, wearable computers,lightweight displays, higher-efficiency emissive displays, large-areadisplays, and improved solar cells.

Current p-TC films have several problems. Most materials that exhibitp-TC behavior are poor conductors with typical conductivities of <1S/cm. The best current p-TC films have conductivities as high as 220S/cm at room temperature, but these films require high processingtemperatures (at least 400° C.). Therefore, they are incompatible withorganic optoelectronic materials and exhibit interdiffusion when heatedwhile in contact with other inorganic films. In addition, many existingmethods of forming high-conductivity p-TC films require the use ofsingle-crystal substrates such as MgO, making them impractical for mostapplications.

Barium copper sulfur fluoride (BCSF) is a crystalline material that, inthe prior art, consists of alternately stacked fluorite andanti-fluorite layers that form a tetragonal crystal structure in spacegroup P4/nmm. Bulk BCSF compound is intrinsically p-type, and when dopedwith potassium, it has been shown to possess bulk conductivity as highas 100 S/cm in selected crystals.

One method for producing BCSF thin films is described in the followingreference, the entire contents of which are incorporated herein byreference: Yanagi, Park, Draeske, Keszler, and Tate, “P-typeconductivity in transparent oxides and sulfide fluorides,” J. SolidState Chem., 175, 34-38 (2003) (hereinafter referred to as the Yanagiarticle). As described in the reference, the authors co-evaporated Cumetal and BaF₂ onto SiO₂ and MgO substrates. They then treated the filmsunder flowing H₂S gas. The resulting films had conductivities of only˜0.1 S/cm at room temperature.

Bulk BCSF

There are currently two general methods for fabricating bulk BCSF. Thefirst method entails heating precursors in a sealed quartz ampoule. Asdescribed in Zhu, Huang, Wu, Dong, Chen, and Zhao, “Synthesis andcrystal structure of barium copper fluorochalcogenides: [BaCuFQ (Q=S,Se)],” Materials Research Bull., 29, 505-508 (1994), the entire contentsof which are incorporated herein by reference, precursors CuS, BaS, Cu,and BaF₂ were pressed into pellets and heated to 450° C. for 12 hours ina sealed quartz ampoule. The second method does not make use of a sealedampoule. Some examples of the second method are described in the Yanagiarticle. In one example, the precursors Cu₂S, BaS, and BaF₂ were heatedto 450° C. for 15 hours. In another example, BaCO₃, Cu₂S, and BaF₂ areused as precursors. In these examples, the precursors were heated to550° C. for an unspecified period of time under flowing H₂S gas in arefractory boat rather than a sealed ampoule. The following article, theentire contents of which are incorporated herein by reference, describesanother example in which precursors BaCu₂S₂ and BaF₂ were heated to 650°C. for 15 hours: Park, Keszler, Yanagi, and Tate, “Gap modulation inMCu[Q_(1-x)Q′_(x)]F (M=Ba, Sr; Q, Q′=S, Se, Te) and related materials,”Thin Solid Films, 445, 288-293 (2003).

None of these methods produces good quality BCSF. The quartz ampoulemethod results in residual quantities of the precursors within the bulkmaterial. This effect may be mitigated by pressing the precursors intopellets prior to baking. Although this technique helps, it does notsolve the problem. In addition, this technique adds an extra processingstep and may be impractical for large quantities of material.Furthermore, placing the precursors directly in contact with the quartzampoule can result in chemical reactions between the precursors and theampoule and lead to contamination with oxides. The second methodrequires treatment under flowing H₂S gas—a step that is potentiallyhazardous and may be impractical for large quantities of material.

P-TC Thin Films on Plastic Substrates

Both p-TC's and TC's with n-type charge carriers (n-TC's) are desired inorder to form a variety of transparent electronic and opto-electronicdevices. While several n-TC's that can be deposited on plasticsubstrates are well-established, such as indium tin oxide, zinc oxide,and amorphous In—Ga—Zn—O, there are no available p-TC's with comparableconductivities.

Currently, there are only a few p-TC's that are compatible with plasticsubstrates. One existing approach is to use organic thin films. Whilethis approach has resulted in both n-TC's and p-TC's, theirconductivities are generally lower than desirable, and these materialsare typically subject to environmental degradation. Another possibleapproach is to use an inorganic material. Most existing inorganic p-TC'srequire deposition at high temperatures (>300° C.). Some requiresingle-crystal substrates that act as a template for crystalline filmgrowth. Either of these requirements makes the use of plastic substratesimpossible with the exception of the amorphous semiconductor,a-ZnO.Rh₂O₃, as reported in Harushima, Mizoguchi, Shimizu, Ueda, Ohta,Hirano, and Hosono, “A p-type amorphous oxide semiconductor and roomtemperature fabrication of amorphous oxide p-n heterojunction diodes,”Advanced Materials, 15, 1409-1413, the entire contents of which areincorporated herein by reference. However, these films possesselectrical conductivity of only 2 S/cm, approximately three orders ofmagnitude lower than that of indium tin oxide. Higher conductivityp-TC's are needed for efficient, low operating voltage devices.

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention whichprovides a bulk barium copper sulfur fluoride (BCSF) material made bycombining Cu₂S, BaS and BaF₂, heating the ampoule between 400 and 550°C. for at least two hours, and then heating the ampoule at a temperaturebetween 550 and 950° C. for at least two hours. The BCSF material may bedoped with potassium, rubidium, or sodium. The present invention alsoprovides for a BCSF transparent conductive thin film made by forming asputter target by either hot pressing bulk BCSF or hot pressing Cu₂S,BaS and BaF₂ powders and sputtering a BCSF thin film from the targetonto a substrate. The present invention is further directed to a p-typetransparent conductive material comprising a thin film of BCSF on asubstrate where the film has a conductivity of at least 1 S/cm. Thesubstrate may be a plastic substrate, such as a polyethersulfone,polyethylene terephthalate, polyimide, or some other suitable plastic orpolymeric substrate.

The present invention has many advantages over the prior art. For bulkBCSF, by placing the precursors within a vitreous carbon crucible, thereactants are isolated from the quartz ampoule which preventscontamination with oxides from the ampoule. Additionally, the two-stepbaking schedule results in negligible amounts of residual unreactedprecursors, such as BaF₂, which eliminates the need for pressing thematerial into pellets before baking or sulfurdizing the material underflowing H₂S and thus considerably lowers processing costs. In contrast,BCSF produced by existing methods may contain more than 5 mole percentunreacted BaF₂ or may require pressing the material into pellets priorto baking or sulfurdizing the material under flowing H₂S. Moreover, fordoped BCSF, the two-step baking schedule may allow the KF or otherdoping material to be incorporated into the material rather thandeposited on the walls of the ampoule.

For BCSF thin films, the method of the present invention may result inan improvement in conductivity of at least two orders of magnitude overexisting methods. Also, the proposed method is compatible with a largevariety of crystalline and non-crystalline substrates. Additionally, theBCSF films exhibit transparency from the visible to the infrared portionof the electromagnetic spectrum out to a wavelength of at least 13 μm.

For BCSF thin films on plastic substrates, the method of the presentinvention permits deposition at low temperatures that is compatible withplastic substrates. Additionally, the films possess higher conductivitythan other p-TC's that are compatible with plastic substrates. Moreover,BCSF films may possess better environmental stability than typicalorganic transparent conductive thin films.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the x-ray diffraction (XRD) data for undoped BCSF.

FIG. 2 is a plot of the transmittance of a 250 nm thick BCSF film on a250 μm thick fused silica substrate. The transmittance of the blanksilica substrate is also shown. The data for wavelengths <3.3 μm, to theleft of the dotted line, were obtained with a spectrophotometer, and thedata for wavelengths >3.3 μm were obtained with an FTIR system. Theinset shows the transmittance of a 150 nm thick BCSF film on a 1 mmthick ZnS substrate demonstrating transparency to at least 13 μm. Thetransmittance on ZnS is higher when the film is present because the filmacts like an anti-reflective coating.

FIG. 3 contains Hall measurement data for a 380 nm thick BCSF film. Thea) conductivity, b) carrier density, and c) calculated mobility areshown as a function of temperature.

FIG. 4 shows powder XRD data for a 10 at. % K-doped bulk BCSF (upperplot) and for a 2.5 at. % K-doped BCSF film (lower plot).

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to a method of fabricating bulkBCSF. It also relates to BCSF films that exhibit conductivities at roomtemperature of at least 1 to 10 S/cm, which is at least 10 times higherthan existing methods. They are typically deposited at 100° C. but canbe deposited at temperatures as low as room temperature. A 250 nm thickfilm on a silica substrate exhibits a peak transmittance of about 80% inthe visible region of the electromagnetic spectrum. Moreover, thepresent invention allows the film stoichiometry to be carefullycontrolled by controlling target composition.

One embodiment of the present invention is for fabricating BCSF films byhot pressing the starting materials into a sputtering target. Thesputtering target can be hot pressed from bulk BCSF compound or it canbe hot-pressed from BCSF precursors, skipping the step of making bulkBCSF compound.

Another embodiment of the present invention is for increasing theconductivity of BCSF films with a post-deposition water treatment.Typically, the films are soaked in water and baked at a temperature ofat least 80° C. Films with conductivities as high as 800 S/cm have beenmade with this method.

A further embodiment of the present invention is for the formation ofp-TC films of BCSF on plastic substrates. BCSF films with conductivityas high as 134 S/cm have been deposited on polyethersulfone (PES)substrates at a temperature of 100° C. during deposition. Thisconductivity is a factor of more than 65 times that of prior methods.Films may be formed on other plastics, such as polyethyleneterephthalate (PET) and polyimide substrates, at room temperature.

Another embodiment of this invention is for BCSF films and bulkmaterials. Alternatively, BaCuQF (where Q=Se, Te) may be fabricated bythe same method. Also, SrCuQF (where Q=S, Se, Te) may be fabricated bythe same method. Additionally, to alter the bandgap of the material,some combination of S, Se, and Te could be used to formBaCuS_(x)Se_(y)Te_(1-x-y)F or SrCuS_(x)Se_(y)Te_(1-x-y)F.

Target Fabrication from Bulk BCSF Compound

To fabricate undoped bulk BCSF, the precursors Cu₂S, BaS, and BaF₂ arebatched in stoichiometric proportions in a nitrogen-purged glove box.The powders are ground and mixed thoroughly until the mixture appearshomogeneous. The mixture is placed in a vitreous carbon crucible insidea quartz ampoule, and the ampoule is sealed under vacuum.

The sealed ampoule is placed in a furnace and is heated in a two stepprocess. The first step is a 400-550° C. bake for at least two hours,preferably using a temperature between 450 and 500° C. for about tenhours. The second step is a higher temperature (550-950° C.) bake for atleast two hours, preferably using a temperature between 600 and 700° C.for about 95 hours. This step drives the reaction to completion,consuming the precursors. Optionally, a small quantity of sulfur (around0.1 percent of the mass of the batch or greater) could be placed withinthe ampoule but outside of the carbon crucible so that during baking, asulfur atmosphere is present within the ampoule. This may help tocontrol sulfur content in the product. If making Se and/or Te containingmaterial, then Se and/or Te can be placed in a similar location toprovide their respective atmospheres.

To fabricate doped BCSF the same processing method described above isfollowed except that K, Rb, or Na is substituted for Ba by including KF,RbF, or NaF in the batch and adjusting the proportion of BaF₂ tomaintain approximate stoichiometric proportions. For example, 2.5 to 10atomic percent of K can be substituted for Ba by including KF in thebatch and adjusting the proportion of BaF₂. Alternatively, K₂S, Rb₂S, orNa₂S can be included in the batch and the proportion of BaS adjusted tomaintain approximate stoichiometric proportions. As an alternative, thedopant may be uniformly mixed with un-doped BCSF prior to hot pressinginstead of forming doped BCSF and then hot pressing the material.

FIG. 1 shows an x-ray diffraction plot of an undoped BCSF sampledemonstrating that the sample is predominantly the BCSF phase. Inaddition, no SiO₂ or other oxide impurities were observed. The bulk BCSFwas then hot pressed into a 3 inch diameter disk that could be used as asputtering target.

It was observed that if only the low temperature bake was used, a largeamount (approximately 50 mole %) of the BaF₂ remained unreacted. If onlythe second step was used, a somewhat smaller but still significantamount (approximately 15 mole %) of the BaF₂ remained unreacted. Inaddition, if only the high temperature step was used when making K-dopedBCSF, much of the KF failed to be incorporated into the material and wasinstead deposited on the walls of the ampoule. When both baking stepswere used, an insignificant amount of the BaF₂ remained unreacted andthe KF was successfully incorporated into the material.

Target Fabrication from BCSF Precursors

A sputter target can be formed by hot pressing Cu₂S, BaS, and BaF₂powders (in stoichiometric proportions) into a disk (such as a threeinch diameter disk), which can be done under vacuum. To create p-dopedmaterial, K, Rb, or Na is substituted for Ba by including KF, RbF, orNaF in the batch and adjusting the proportion of BaF₂ to maintainapproximate stoichiometric proportions. Alternatively, K₂S, Rb₂S, orNa₂S can be included in the batch and the proportion of BaS adjusted tomaintain approximate stoichiometric proportions. For example, thefollowing amounts were used for a 4 gram batch: 1.27847 g Cu₂S, 1.267 gBaF₂, 1.3607 g BaS, and 0.0933 g KF.

Alternatively, rather than using one target that includes all of theprecursors, multiple targets—each comprising one or more of theprecursors—may be used. In this case, the power to each target could beadjusted independently so that the relative sputter rate of each couldbe controlled. This would allow for control of the stoichiometry of thefilm without the need for fabricating a new sputtering target.Additionally, this method may potentially increase the film'sconductivity.

The duration and temperature of the hot pressing process are carefullychosen so that sufficient chemical bonds form to make the target solidand robust. They are intentionally chosen, however, to be small enoughso that an insignificant amount of BCSF forms due to a solid statereaction. As a result, the target consists of an intimate mixture of theprecursor powders. The fraction of Ba, Cu, S, F, and K can be altered byadjusting the ratio of the precursors.

A sample processing schedule using Cu₂S, BaS, BaF₂, and KF precursors isas follows: ramp at a rate of 5° C./min. to 550° C.; apply pressure of3,600 psi and hold for about an hour; ramp down to 470° C. and releasepressure; hold for about 2 hours (to allow target to anneal); and thenramp at 1° C./min. to room temperature.

BCSF Film Fabrication

Deposition can be carried out by RF magnetron sputtering in a sputter-upgeometry onto a substrate. The deposition system used had a basepressure of 1×10⁻⁷ T. Deposition was carried out in an Ar atmospherewith a pressure of 5 mT and an Ar flow rate of 20 sccm. The substratetemperature ranged from room temperature up to 250° C., with the bestresults being obtained at a temperature around 100° C. RF power ofapproximately 50 W was used, resulting in an energy density ofapproximately 1 W/cm². The resulting deposition rate was 9 Å/min.Deposition times were up to 15 hours. A typical sample is as follows: 2inch diameter silica sample, substrate temperature of 100° C., RF powerof 50 W, deposition time of 7 hours, resulting sample thickness of 350nm. Annealing of films in vacuum, in an inert atmosphere, or underflowing H₂S gas may be used to increase film conductivity.

The resulting films exhibited good transparency. FIG. 2 shows thetransmittance of a 250 nm thick BCSF film on a 250 μm thick fused silicasubstrate. The peak transmittance is approximately 80% at a wavelengthof 700 nm. The inset shows the infrared transmittance of the film on aZnS substrate. As shown on the plot, the film transmits out to awavelength of at least 13 μm where the transmittance is limited by thatof the ZnS substrate.

FIG. 3 shows the electrical properties of 150 nm thick BCSF films. Datafor both an as-deposited sample and a water-treated sample are shown oneach plot. The water treatment consisted of soaking the sample indeionized water for 3-5 minutes, followed by a bake at 100° C. for atleast 5 minutes. In plot a), the conductivity as a function oftemperature is shown. The conductivity of the as-deposited sample isapproximately 180 S/cm at T=300 K. Water treatment further enhances theconductivity by a factor of 3 to 4. The conductivity of thewater-treated sample was approximately 800 S/cm at T=300 K. Plot b)shows the carrier concentration, and plot c) shows the hole mobility asa function of temperature. The carrier concentration is approximately1×10²¹ for the as-deposited sample and 4×10²¹ for the water-treatedsample. The mobility is approximately 1.2 cm²/Vs for both samples,although the variation as a function of temperature is greater for theas-deposited sample. It is evident from this data that the enhancementin conductivity for the water-treated sample is primarily a result of anincrease in the concentration of charge carriers.

Unlike the prior art, the films consist of a cubic phase rather than aP4/nmm phase. The phase of bulk BCSF and that of a film were compared byuse of XRD. The upper plot in FIG. 4 shows powder XRD data for a 10 at.% K-doped bulk BCSF, and the lower plot shows data for a 2.5 at. %K-doped BCSF film. Note that, while the dopant concentration isdifferent for the bulk and film samples shown, the XRD spectrum does notvary significantly with dopant concentrations in the 0-10 at. % range.This is true for both bulk and film samples. The data for bulk BCSFshows that the material is primarily the P4/nmm phase. Only a smallamount of residual unreacted BaF₂ was observed.

The crystalline phase of the film is different than that of the bulkBCSF. The peaks for the P4/nmm phase are completely absent, while thenew peaks are consistent with a face-centered cubic structure with alattice parameter of α=6.27 Å. Two of the precursors also haveface-centered cubic structure—BaF₂ and BaS—with lattice parameters of6.20 Å and 6.39 Å respectively. The location and relative intensities oftheir peaks are shown in FIG. 3. The lattice parameter of the phasefound in the BCSF film lies between these two. Based on a comparison ofthe intensity ratios for the BaF₂ and BaS FCC phases, we can infer thatthe space group of the observed cubic phase of BCSF is Fm3m.

BCSF on Plastic Substrate

Deposition can be carried out by RF magnetron sputtering in a sputter-upgeometry onto a plastic substrate. The substrate was clipped onto ametal substrate holder that rotated during deposition to provide gooduniformity. This process was carried out in an Ar atmosphere with apressure of 5 mT and a flow rate of 20 sccm. An energy density ofapproximately 1 W/cm² was used. The resulting deposition rate was 9Å/min. Films were deposited on PES substrates at a substrate temperatureof 100° C. For deposition onto PET and polyimide substrates, depositionwas carried out at room temperature.

The resulting films were transparent from approximately 400 nm into thenear-infrared. The peak transmittance was approximately 55% at awavelength of 640 nm. The transmittance falls off to a level of 10-15%throughout the near-infrared.

The conductivity of the films was measured at room temperature with afour-point probe. The conductivity of films on PES substrates was ashigh as 134 S/cm. The conductivity of the films on the PET and polyimidesubstrates was approximately 1 to 10 S/cm.

To determine the sign of the carriers, films were deposited on fusedsilica substrates using the same processing conditions used with theplastic substrates. The sign of the Seebeck coefficient was evaluated byapplying a thermal gradient to the film and measuring the voltagebetween two probes at either end of a film. The sign of the Hallcoefficient was also measured. Both measurements confirmed that thecharge carriers are p-type, so it can be inferred that the films onplastic substrate also possess p-type conductivity.

Any plastic or polymeric substrate may be used. An opaque plasticsubstrate may be used for applications that do not require transparencyyet benefits from other properties of plastic substrates such as lowcost, light weight, or flexibility.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method for making a barium copper sulfurfluoride (BCSF) transparent conductive thin film, comprising: (a)forming a sputter target by hot pressing bulk BCSF; and (b) sputtering aBCSF thin film from the target onto a substrate; wherein aftersputtering, the film is soaked in water for at least 30 seconds and thenheated at 100° C. for 5 minutes; and wherein the resulting film has aconductivity of 800 S/cm.
 2. The method of claim 1, wherein the BCSFcrystalline phase is cubic in space group Fm3m.
 3. The method of claim1, wherein the substrate is a plastic substrate.
 4. The method of claim3, wherein the plastic substrate is selected from group consisting ofpolyethersulfone, polyethylene terephthalate, and polyimide substrates.5. A method for making a barium copper sulfur fluoride (BCSF)transparent conductive thin film, comprising: (a) forming a sputtertarget by hot pressing Cu₂S, BaS and BaF₂ powders in stoichiometricproportions; and (b) sputtering a BCSF thin film from the target onto asubstrate; wherein after sputtering, the film is soaked in water for atleast 30 seconds and then heated at 100° C. for 5 minutes; and whereinthe resulting film has a conductivity of 800 S/cm.
 6. The method ofclaim 5, wherein the BCSF crystalline phase is cubic in space groupFm3m.
 7. The method of claim 5, wherein the substrate is a plasticsubstrate.
 8. The method of claim 7, wherein the plastic substrate isselected from group consisting of polyethersulfone, polyethyleneterephthalate, and polyimide substrates.